Pat the sirloin steak dry with paper towels and season both sides evenly with sea salt and black pepper.
Heat one teaspoon of olive oil in a cast-iron skillet or heavy pan over medium-high heat until it begins to shimmer.
Place the steak in the hot pan and sear for approximately 3 to 4 minutes per side for medium-rare, or until it reaches your desired doneness.
Transfer the steak to a cutting board and allow it to rest for at least 5 minutes to keep the juices intact.
While the steak rests, dice the cucumber into bite-sized pieces, halve the cherry tomatoes, and thinly slice the red onion.
In a small bowl, whisk together the remaining teaspoon of olive oil, red wine vinegar, and dried oregano to create the dressing.
In a large bowl, combine the cucumber, tomatoes, onion, and crumbled feta cheese, then toss with the prepared vinaigrette.
Slice the rested steak against the grain and serve it immediately on a plate alongside the fresh cucumber salad.